The invention relates to a method for manufacturing an air-gap-insulated exhaust manifold.
A method for manufacturing an air-gap-insulated exhaust manifold is known from German Patent Document DE 195 11 514 C1. The exhaust manifold in that document is composed of three inner tube sections as well as an upper shell and a lower shell that form the outer shell. The inner tube sections, consisting of a 90.degree. curved tube, a tee, and an intake tube section, are inserted into one another in the above sequence on the assembly line with a sliding fit, with the tee in each case having one end of its cross beam connected with the other two tube sections. The ends of the tube sections that are not connected with one another lie parallel to one another in a plane and each have flanges for bolting to a cylinder head of an internal combustion engine. The intake tube section, made approximately in the shape of a Y, has also a second free end that is not aligned parallel to the other ends of the remaining inner tube sections, but extends in a direction opposite to them. An outlet flange is mounted at the second free end of the intake tube section, to which an exhaust line, possibly with a catalytic converter, can be connected. For preliminary centering of the inner tube sections during assembly inside the later outer shell, forming the desired uniform air gap, before the parts are inserted into one another, spacing rings are pushed onto the individual sections. These rings are made of a material that dissolves during later operation of the exhaust system as a result of the heat of the exhaust, so that the inner tube sections can expand freely radially under the alternating thermal stresses produced by operation. The inner tube sections are placed in the lower shell in the inserted position, and then the upper shell is placed on the lower shell and pressed against it by a mechanically controlled press. Then the inner tube sections are welded to the mouth openings at the outer shell, with centering being retained. Then the upper shell is welded to the lower shell at the edges.
The overlapping areas of the inner tube sections are generally short in terms of their lengthwise extent because of the dimensions of the section ends, so that the sections are relatively movable in the outer shell and the curved tube in particular can be loosened from the shell without difficulty. Because of the loose connection of the inner tube sections to one another by the desired sliding fit in the plug connection, with the degree of looseness depending to a large degree on the manufacturing tolerances of the inner tube sections in the radial direction, it can happen that as a result of vibration of the parts during assembly, for example during manual mounting of the upper shell on the lower shell and during subsequent mechanical pressure, as well as by the centrifugal forces that occur during the pivoting movement of the turntable during the transfer of the assembled part from the assembly station to the welding station, the inner tube sections shift, causing the plug connections to come loose. This occurs in particular with the curved tube section that can almost twist free of the connection.
As a result of the loosening of the plug connection, the continuous flow of gas is disrupted, which later results in considerable problems of the exhaust flow during driving and as a result of the sharp increase in flow resistance that takes place, leads to high engine power losses. Similarly, the air gap insulation is eliminated since the hot exhaust can flow unimpeded through the outer shell. Moreover, the loose ends of the inner tube sections can cause irritating rattling sounds during driving, whenever sections strike the outer shell.
Furthermore, welding the curved tubes to the inside of the outer shell is highly problematic, since the curved tube is positioned at an angle as a result of its rotation out of the plug connection at the mounting end, so that the welding robot which cannot detect such deviations in the relative positions of the two welding connection partners from the programmed required range, does not act at the correct position, so that the welded connection, if it is indeed produced in some way, cannot withstand any mechanical stresses. Moreover, the curved tube can be loosened from the plug connection to an extent such that it covers the correspondingly designed outlet of the outer shell with its end, so that a hollow welded seam is no longer possible so that the curved tube cannot be attached to the outer shell. Of course it is possible for the fitter to compensate for the manufacturing tolerances of the inner tube sections as far as the loosening of the plug connection is concerned by carefully varying the depth of penetration, so that the manufacturing process described above takes place completely and with perfect qualitatively. Usually however a there is not sufficient time for the necessary care because of shorter processing cycle times. In addition, human error can always be expected, which means that the inner tube sections are inserted for much too short a distance into one another.
A goal of the invention is to improve on a method of the above-discussed type such that, in a simple fashion, a sliding fit of the inserted inner tube sections, as well as the formation of a uniform air gap insulation of the inner tube sections following completion of the manufacture of the exhaust manifold is always guaranteed.
This goal is achieved according to the invention by providing a method for manufacturing an air-gap-insulated exhaust manifold, said manifold being formed of an exhaust-conducting inner tube assembled from individual sections and an outer shell divided into a upper shell and a lower shell, designed to match the shape of the inner tube, with the inner tube sections, of which at least one is designed as a curved tube, being inserted into one another and in which the inside diameter of the respective receptacle of one tube section is larger than the outside diameter of the tube section end of the other tube section to be inserted into this receptacle, being inserted centered as a assembled part into the lower shell of the outer shell, with the upper shell then resting on the lower shell and being pressed against the latter, and with welding of the upper and lower shells and the inner tube sections with the outer shell then being performed in the vicinity of their outlet openings, characterized in that inner tube sections before the insertion of inner tube into lower shell are connected forcewise with one another by means of a connecting means made of heat-volatile material, with the connecting means, following welding of the upper and lower shells with the inner tube sections, being dissolved under the influence of heat.
By virtue of the invention, following the assembly process, a plug connection is made possible that is initially rigid and is later movable during driving within the scope of the play and thermal expansion and/or contraction of the inner tube section. The background of an initially rigid plug connection lies in the exact location and alignment of the inner tube sections with respect to one another as well as of the inner tube sections with respect to the outer shell. The former is necessary because otherwise, during the operation of the exhaust system, the inserted ends of the inner tube sections in their active receptacles become tilted, so that the desired sliding fit of the plug connections is suppressed. However, this is unavoidable for compensation of the relative movement of the plug partners which takes place as a result of changing thermal loads. The latter is necessary for the uniformity of the insulating air gap between the outer shell and inner tube.
In addition, manual assembly errors and shifting of the plug partners with respect to one another, which arise from mechanical pivoting movements and vibrations during mounting and installation of the upper shell, with the respective insertion depth becoming unacceptably short, is avoided by forcewise connection. The limited insertion depth leads to tilting of the plug partners as a result of the inserted plug partner sliding out. In addition, the slip-free connection achieved by the invention permits exact positioning of the end of the curved tube end of the inner tube at the outer shell, so that optimum welding can be performed between the outer shell and the inner tube in this area. When this welding is performed, the inner tube remains centered immovably in the outer shell and remains aligned optimally between the inner tube sections with respect to a perfectly functioning sliding fit. In order to produce a sliding fit, the rigid plug connection must be released after welding. This is accomplished simply and advantageously by the action of heat, causing the connecting means which previously provided the rigid connection to melt. Thus, the invention firstly guarantees a functioning sliding fit once manufacture of the exhaust manifold is complete and secondly guarantees uniform air gap insulation. In addition, the inner tube, assembled separately, can be delivered as a preassembled component to the assembly line, saving production time. The preassembled structural unit need then only be inserted into and welded to the outer shell.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.